Magnesium alloys, which have a high specific strength, are the lightest of alloys, are applicable in a variety of casting and machining processes, and have a wide range of application, and are thereby used in almost all fields in which light weight is required, such as parts for vehicles and electronic parts. However, magnesium (Mg) is a metallic element that has a low electrochemical potential and is very active. Mg still has limitations in terms of the stability and reliability of the material, since it undergoes a strong reaction when it comes into contact with oxygen or water, and sometimes causes fires. Therefore, the fields in which Mg can be applied are still limited compared to its potential applicability. In particular, it cannot be used in applications in which safety is important.
Because of this activity of Mg alloys, it is necessary to create an inert atmosphere using an inert mixture gas, such as a flux or CO2+SF6. Since the flux that is used in melting and refining is a chlorinated substance, there is a problem in that chlorine atoms reside inside a material, thereby significantly decreasing corrosion resistance when the conditions for processing the molten metal are not fulfilled. In order to solve this problem, it is effective to perform melting and casting in an atmosphere in which SF6, CO2 and air are mixed, instead of using the flux. However, SF6 is classified as a greenhouse gas, the global-warming potential (GWP) of which is 24 times that of CO2, so that the use thereof is expected to be regulated in the future time.
In order to more fundamentally solve this problem, studies for improving the oxidation resistance of Mg alloys, in particular, studies intended to increase the ignition temperature of Mg alloys by adding Ca, Be or rare-earth metals, have been carried out. Traditionally, Ca has been a main choice among the alloying elements that are added to Mg alloys that are oxidation resistant because Ca is cheaper than other rare-earth metals, is nontoxic, and greatly increases the ignition temperature in consideration of the amount that is added.
According to previous studies on magnesium alloys that contain Ca, it is known that the ignition temperature increases by about 250° C. when 3 wt % or greater of Ca is added. Therefore, the ignition temperature should be maintained as higher as possible in order to stably cast Mg alloys, which contain Al of 7 to 11 wt %, for example, without a shielding gas. To this end, it is preferred that a great amount of Ca be added to Mg alloys.
However, when a great amount of Ca is added particularly in an amount greater than 2 wt %, the tensile properties of Mg alloys are generally degraded, with the decrease in elongation being particularly significant. This is because a great quantity of coarse and brittle eutectic phases is formed, thereby resulting in cracks. In addition, when Ca is added in an amount greater than 2 wt %, there occurs a problem of die sticking, making it difficult to manufacture a product. Therefore, there is the demand for the development of a magnesium alloy that does not cause other problems such as sticking or the like while satisfying both the ignition resistance and the tensile properties.